event-loop: Handle EINTR and EAGAIN in wl_event_loop_dispatch

This fixes an issue where it was not possible to start Gamescope under GDB on some setups. https://github.com/ValveSoftware/gamescope/issues/743

Any signals would cause epoll_wait to return -1 and set errno to EINTR.

This also handles the EAGAIN case like the other polling loops in libwayland.

Signed-off-by: Joshua Ashton joshua@froggi.es